Drag "ARCamera" from Assets tab to "Camera" gameobject to the left in Hierarchy tab

Drag "ARTrackedObject" from Assets to "Marker" gameobject in Hierarchy tab

Adding Marker:

We will be working with the markers already provided by ARtoolkit library. If you want to create your own marker read this article Create Custom Marker

Marker that I have used is

In Hierarchy tab, select "ARtoolkit" option and in Inspector tab goto "ARmarker" Script, give your marker tag an unique name which you can easily remember and use it again. In below image, I have named my Marker tag as "marker1"

Select "Marker" from Hierarchy and in inspector tab goto "AR Tracked Object" Script, give Marker tag same name as you have given above. In my case, I will name it as "marker1"

As soon as you type "marker1" in your marker tag textbox, you can see "Got Marker" status changes to "Yes". If not, follow above steps to check whether your script is added properly and make sure marker tag name in "ARMarker" and "AR Tracked Object" is same

Note:

If you have created a custom marker, put the pattern file in "Assets/ARToolKit5-Unity/Resources/ardata/markers" location. And in ARToolkit script and ARMarker script change the pattern file name to what you have inserted in "Assets/ARToolKit5-Unity/Resources/ardata/markers" location.

Adding Object that you want to display on Augmentation:

I am adding a Simple 3D "sphere" you can add anything you want like cube, capsule or any animation.